A superconducting technology machine has a stator, which encloses a rotor mounted so as to rotate about a rotor axis while forming an air gap, wherein the rotor contains a cold part, in which superconductors of a rotor winding, which are to be cooled, are arranged and a which is enclosed by a warm housing on whose outer wall, facing the air gap, there is at least one part acting as a dampener shield, and has thermal insulation between the cold part and the warm housing.
A corresponding machine is disclosed by U.S. Pat. No. 4,914,328 A.
So-called sudden short-circuit represents a serious anomalous situation of rotating electrical machines. In the event of such a sudden short-circuit, overload torques can occur which amount to a multiple of the net torque of the machine, for example from 10 to 20 times in the case of a two-pole rotor winding. Despite all precautionary measures, during the operation of a machine it is not possible to rule out such short-circuits which, for example, may be due to misconnections or incorrect servicing. As a corresponding precautionary measure, a type test of a machine furthermore often requires demonstration that such anomalous situations will not cause long-term damage to parts of the machine.
Besides other undesired effects in such anomalous situations of a machine with a superconducting rotor winding, considerable loads also occur in the radial outer wall of a warm housing of the rotor, which is formed as a vacuum housing or cryostat housing for thermal insulation reasons. This is because the housing or cryostat outer wall which faces the stator winding, and which generally is formed of a metallic material, simultaneously acts as an electromagnetic dampener shield or carries such a shield (cf. U.S. Pat. No. 4,914,328 A cited in the introduction). Heavy currents can moreover be induced here, especially when the dampener shield is formed of a material with good electrical conductivity. In interaction with the magnetic B field generated between the rotor and the stator, these currents then entail high Lorentz forces.
As mentioned, such loads have twofold effects on a dampener shield. On the one hand, the dampener shield is deformed by the forces occurring. There are regions where it is exposed to a compression force from the outside and bends inward, while in other regions it experiences a corresponding deformation outward. The dampener shield must not touch the stator, since the latter could then be destroyed. Assistance can be achieved here by an air gap correspondingly dimensioned amply between the stator inner radius and the housing outer radius. The effect of this, particularly in the case of axially extended rotors, is that values are required for the radial extent of the air gap which are significantly more than a few millimeters, as is usual in conventional machines. On the other hand, in such an anomalous situation material stresses also occur in the dampener shield owing to the deformation. The corresponding loads must be less than the limit value for plastic deformation of the selected dampener shield material, since otherwise the outer housing of the rotor would remain deformed after a short-circuit. Assistance can be provided here only by a sufficient wall thickness of the dampener shield.
Both precautionary measures, however, mean that the “magnetic air gap” (the electromagnetic interaction between the rotor winding and the stator winding of the machine) must therefore correspondingly be dimensioned sizably. Since on the other hand the fields decrease with a decreasing distance from the field-generating cold part of the rotor, or its superconducting rotor winding, larger wall thicknesses and large air gaps entail significantly inferior utilization of the superconductor material being employed.